1. Field of the Invention
The present invention relates to a power steering apparatus mounted on a vehicle, for instance.
2. Description of the Related Art
A general power steering apparatus is equipped with a torque sensor for detecting a steering force exerted by a driver, a motor for supplementing the steering force, an inverter for driving the motor, and a target current calculating unit for calculating a target current caused to flow through the motor in accordance with an output of the torque sensor.
The inverter, which is composed of switching elements, controls the driving of the motor through PWM-modulated voltage pulses with a constant frequency.
A conventional electric power steering apparatus is equipped with a motor driving unit (an inverter). In the motor driving unit, while a power source is connected between input terminals of a bridge circuit composed of four pairs of field effect transistors, an electric motor (i.e., a motor) is connected between output terminals of the bridge circuit. The power steering apparatus causes a current to flow through the motor by the motor driving unit to apply a motor power to a steering system. A switch unit is interposed between the motor driving unit and the motor or between the power source and the motor driving unit (e.g., see JP 07-96387 B).
In the aforementioned conventional apparatus, the switch unit is opened to shut off the inverter and the motor from each other or the power source and the inverter from each other when an ON-malfunction occurs in the field effect transistors constituting the bridge circuit.
As described above, the switch unit is opened to shut off the power source and the motor from each other, and a closed-loop circuit is opened to make a transition from power steering to manual steering. Thus, a deterioration in follow-up performance or in steering feeling is prevented.
In the conventional electric power steering apparatus, abrupt disturbances resulting from irregularities of a road surface or the like are directly transmitted to a driver in the event of a transition from power steering to manual steering, so there is caused a problem in that a deterioration in steering feeling cannot be prevented sufficiently.
When the transition from power steering to manual steering is made while the driver exerts a steering force, the motor suddenly stops supplementing the steering force. Therefore, there is also caused a problem in that a deterioration in steering feeling cannot be prevented sufficiently.